Earthquake Updates
Sanitation concerns in post-quake Christchurch
Wendy Zukerman, Australasia reporter
In the New Zealand city of Christchurch authorities are scrambling to restore water supplies and sewage systems which were severely damaged by last week’s 6.3-magnitude earthquake.
Canterbury medical officer of health Alistair Humphrey told New Zealand Doctor that 40 per cent of Christchurch doesn’t have running water and the entire city’s water supply is “compromised”.
(Image: Jamie Ball/Rex Features)
Isolated cases of measles and gastroenteritis have been reported. According to Humphrey the gastro cases were likely to have been water-borne and the result of people brushing their teeth with contaminated water – rather than spread through human contact.
But, a Canterbury District Health Board spokeswoman told the New Zealand Herald: “There is an underlying potential for there to be a measles outbreak. There’s a chance of an outbreak of gastro diseases.”
Many residents are living in camps, where the poor sanitation and cramped living conditions are perfect for disease outbreaks.
On Friday, Cowles Stadium welfare centre – which provided accommodation for Christchurch earthquake evacuees – was forced to close because its water and sewage services were not considered reliable.
Radio New Zealand reported that the Christchurch City Council was “worried about disease” at the stadium, and said it could not “afford an outbreak of diarrhoea.”
All citizens are being encouraged to boil their water before consuming it.
At 12.51 pm local time today – precisely one week from when the earthquake struck, burying as many as 200 people - the city stood silent for 2 minutes.
Mental health is seen as a growing concern in the city, too. A doctor from a nearby hospital that has been helping patients told the New Zealand Herald, “We had walking wounded coming in initially on Tuesday – people with cuts, minor injuries and things like that. We are starting to get more people with shock coming in and I expect that to increase.”
http://www.newscientist.com/blogs/shortsharpscience/2011/03/sanitation-concerns-in-post-qu.html
The tectonic forces that are shredding New Zealand
The week of 22 February the New Zealand city of Christchurch felt the force of a 6.3-magnitude earthquake. The quake came just five months after an even larger one struck 40 kilometres west of Christchurch, near the town of Darfield. In fact New Zealand experiences around 14,000 tremors each year, although most are too small to be felt. They are a sign of the tectonic processes that are gradually shredding the country.
Why is New Zealand so prone to earthquakes?
Regions that lie close to a boundary between tectonic plates tend to feel more quakes than areas in the middle of a plate. New Zealand may have a total land area of just 27,000 square kilometres, but that area happens to coincide with the margin between the Pacific and Australian plates, leaving parts of the island very seismically active.
Which areas are most vulnerable?
Large areas of both North and South Islands have felt earthquakes with a magnitude greater than 5 within the past 200 years. This is because of New Zealand’s unique tectonic regime: despite its small size, the country feels the impact of three distinct regions of tectonic activity.
The relatively low-density continental crust of the North Island, which sits on the Australian plate, is forcing the dense oceanic crust on the Pacific plate beneath it in a process called subduction. This creates a so-called destructive plate margin that is nibbling away at the Pacific plate. Earthquakes are common where a subducting plate grinds against the underside of an overriding plate.
Something similar is occurring to the south-west of South Island. But here the sliver of continental crust lies on the Pacific plate, and it is the Australian plate that is being destroyed through subduction.
In between, the continental crust on the Pacific and Australian plates slide past one another on South Island, creating a conservative plate margin where crust is neither created nor destroyed. This area is still prone to earthquakes, most notably along the Alpine fault. Further away from these fault zones the ground is generally more quiescent. Christchurch is over 100 kilometres from the Alpine fault.
So what caused the Christchurch quake?
It was caused by a new fault – or, to be more precise, a previously unrecognised fault.
“The fault is likely to have existed previously – and possibly produced earthquakes before – but they have not ruptured recently, in a geological sense,” says John Townend at the Victoria University of Wellington, New Zealand. The unrecognised fault appears to be an offshoot from the Alpine fault. Unfortunately for the residents of Christchurch, that offshoot passes very near South Island’s largest city.
Are more quakes on the fault likely?
Earthquake prediction is an inexact science, despite tantalising evidence thatearly warning systems may be possible in some cases. But some seismologists are cautiously optimistic.
“An earthquake of this magnitude does a good job of releasing stress,” says Gary Gibson, a seismologist at the University of Melbourne, Australia. Townend agrees: “My interpretation of what we are seeing near Christchurch is temporary, albeit harrowing, activity in what is generally a relatively low-seismicity part of the broad plate boundary.”
What’s the long-term prognosis for New Zealand?
Even if Christchurch dodges major seismic activity in the near future, tectonic forces will continue to act on New Zealand. Hamish Campbell at the research consultancy GNS Science in Lower Hutt, New Zealand, says it’s “very unlikely” that the newly recognised fault will have any serious effect on the country’s geography, but activity on the Alpine fault may well do so.
The rocks on either side of the Alpine fault are grinding past each other quickly – at around 30 millimetres per year. The southern part of South Island has moved at least 480 kilometres relative to the northern part within the past 25 million years. That rate of movement is “colossal”, says Campbell – and not far off the displacement seen on the world-famous San Andreas fault in California, which is itself a conservative plate margin.
Fast forward several million years and New Zealand will continue to twist and turn. The activity that is already shredding the country will ultimately see South Island “split in two along the Alpine boundary”, says Campbell. The town of Kaikoura would be at the northern tip of one island, with Greymouth at the southern tip of the other, he predicts.
Why this Christchurch earthquake was so shallow
February 22, 2011 5:24PM
GARY Gibson, Principal Research Fellow in the School of Earth Sciences at the University of Melbourne, explains why the 6.3 magnitude earthquake that struck Christchurch today was more destructive than the 7.1 quake that hit the city in September last year.
Just how bad is an earthquake of magnitude 6.3?
Gibson: A magnitude 6.3 earthquake will occur when an active fault area approximately 15km square ruptures, and one side moves about one metre relative to the other.
Its effect depends on how close it is (to a population centre), and ground shaking will be severe within 10 to 20 kilometres of the rupture.
The critical issue with this earthquake was that the epicentre was at shallow depth (5km) under Christchurch, so many people were within 10km to 20km of the fault rupture.
The magnitude 7.1 earthquake on September 4 (last year) was 30km to 40km west of Christchurch and ruptured mainly to the west.
Should we expect further large earthquakes in the area? Are aftershocks likely?
The September earthquake and this earthquake will have relieved the majority of stress in the regions in which they occurred, so another larger earthquake is unlikely.
However, aftershocks will certainly occur over the next few days and weeks which may cause further damage in weakened buildings, and will be very distressing for residents.
Is there a geological reason for multiple large earthquakes occurring within such a short time?
Earthquakes always cluster in time and space with some large earthquakes having foreshocks and most large earthquakes have many aftershocks.
Multiple large earthquakes are not uncommon, often when the main rupture of the earlier event is extended into an adjacent segment of the active fault.
(Continues below)
Why is the New Zealand South Island so geologically active?
New Zealand is on the tectonic plate boundary between the Pacific Plate and the Australia-India Plate.
The plate boundary is east of the North Island and crosses to the west of South Island.
Christchurch is not on the plate boundary, but is near to related secondary faults that result from the bend in the plate boundary to the north.
In the past 200 years, and in the long term, large earthquakes will occur less frequently in Christchurch than along the plate boundary.
However all earthquakes in the Christchurch region will be shallow, so the effect of a given earthquake will be worse than from a deeper plate boundary earthquake of the same magnitude.
How does this rate historically against other earthquakes?
This is by far the largest earthquake to have occurred in the Christchurch region in historic time.
Earthquakes larger than magnitude 6.0, usually deeper than this event, occur about annually in New Zealand, including one of magnitude 7.8 that occurred in the remote southwest of South Island in July 2009 with little damage.
Why is NZ seemingly more prone to earthquakes than Australia? Is a similar earthquake likely to occur in Australia?
New Zealand is more prone to earthquakes because it is on the plate boundary and has many plate boundary earthquakes. Large earthquakes occur infrequently in Australia.
In all of Australia a magnitude 6.0 or larger event occurs on average every ten years.
In the capital cities of Australia, a nearby magnitude 6.0 will occur on average every few thousand years. All earthquakes in Australia are at shallow depth, similar to those in Christchurch.
Is it possible to predict earthquake activity? How much better are we at predicting them and how good can we hope to get?
It is not possible to predict earthquakes, giving location, time of occurrence and magnitude, with certainty.
Aftershocks have continued at a decreasing rate since the September earthquake. Recent aftershocks have been east of the original rupture.
Are there engineering or town planning measures which could be improved to reduce the impact of earthquakes?
Building standards are already very high in New Zealand, but are upgraded as knowledge develops, and as higher standards become economically viable.
Questions and Answers …. the Why’s of New Zealand’s earthquake the way it was..?
New Zealanders living in the city of Christchurch (population 377,000) on the South Island’s Canterbury Plains were hit hard Tuesday by 6.3-magnitude earthquake, an aftershock from September’s 7.1-magnitude quake. Prior to these two seismic events, Canterbury Plains likely had not experienced an earthquake in thousands of years. In fact, scientists did not even know there was a geological fault there until it ruptured last year.
Scientific American spoke with Robert Yeats, a professor emeritus of geology at Oregon State University in Corvallis, about why earthquakes are so difficult to predict and what is being done to even the odds.
[An edited transcript of the interview follows.]
The 7.1-magnitude earthquake in September caught the locals completely off guard. Why was that earthquake, as well as Tuesday’s aftershock, such a surprise?
The earthquakes struck an area of New Zealand’s South Island where sediments are deposited from the Southern Alps and from the nearby rivers. The sediment deposits reach all the way to the east coast on what is called the Canterbury Plains. The fault that ruptured in September had not done so in thousands of years, during which sediments had been deposited on top. I’ve been out there, and it’s like driving anyplace where it’s all flat. There are farms, but there’s nothing that says, “Here’s a fault.” So when the earthquake struck in September, they were totally surprised. On the South Island, the Hope Fault and Marlborough Fault System are better known—there had been an earthquake in 1888 along the Hope Fault. Much less was known about faulting on the Canterbury Plains because no earthquake had happened in their historical record.
The latest Christchurch tremors were not as strong as the original earthquake, but they have caused considerably more damage and claimed dozens of lives (no one died during the September quake). The Christchurch epicenter was only 10 kilometers outside of the city, whereas the 2010 seismic event took place about 40 kilometers to the west, where there is mostly farmland. Adding to Christchurch’s misfortune, the aftershock struck only about four kilometers in depth below the city, whereas September’s seismic event originated about 10 kilometers down. Compounding these problems, Tuesday’s quake hit during lunchtime, when the city was buzzing with activity, whereas the earlier disaster occurred during early morning hours.
What defines an “aftershock” as opposed to an earthquake? Does a certain amount of time have to elapse for a seismic event to be considered an earthquake?
It takes many years before seismic activity can be considered an earthquake rather than an aftershock of a previous earthquake. That’s a point of debate among seismologists. If you look at a map of Southern California, you’ll see quite a few little earthquakes south of Bakersfield in the San Joaquin Valley. Some people regard those as still aftershocks from the earthquake of 1952, which measured 7.3. That’s not the consensus, but it indicates that these aftershocks go on for decades. It takes quite a long time for everything to become quiet again. Now the San Andreas, on the other hand, had a large earthquake in 1857, just west of San Joaquin Valley, and it’s quiet as could be. Same with the area of the 1906 San Francisco earthquake along the San Andreas. You can’t paint all aftershock series with the same brush.
You mentioned that the aftershock was “shallow.” What is the difference between an earthquake that takes place four kilometers below ground and one that takes place 10 kilometers below the surface, as the September earthquake did?
It’s like how close you are to a bomb going off. If you’re within a couple of kilometers, you’re likely to get injured. If you’re three or four times that distance, you not likely to. The waves are attenuating or propagating toward the surface. Christchurch was a very shallow earthquake, and that’s a reason why the damage was much worse than the earlier one.
What do you look for when you investigate seismic activity in a particular area?
If I study a particular fault, I like to know its slip rate, how fast it’s moving, whether it’s a millimeter per year or a centimeter per year. As plates move, they’re building up strain, and I estimate how much strain can build up before there is a rupture. New Zealanders have been good about trenching faults (digging trenches along fault lines to study previous seismic activity). The problem with this fault was that they didn’t even know it was there. That tells me it’s a pretty slow-moving fault but nonetheless when it builds up toward an earthquake of magnitude 7, then that’s going to continue to produce aftershocks for a long time. It’s not an exact science.
I’m working with the New Zealand Institute of Geological and Nuclear Sciences on a project funded by the Global Earthquake Model to map all of the active faults on Earth. This Global Faulted Earth project will include a global active fault and seismic source database along with a book I’m writing. At some point in the not too distant future, if you hear about an earthquake in a place like Christchurch, you will be able to click on this database to find out what is known about it.
What can be learned from this week’s aftershock in New Zealand?
We can map faults and that’s what we do, and we can use what we learn about those faults to establish some probability of an earthquake happening, but you can’t map all faults. In the case of Christchurch, I’m not sure what they could have done differently. They could have said, let’s do a seismic survey of the whole Christchurch metropolitan area just to be sure there’s nothing going on underneath the city. But it was unlikely for the faults to extend as far south from the original Darfield earthquake site at as they did, so I can’t fault them for not doing that.
You have to realize that New Zealand has some of the strongest building codes in the world, and those building codes are respected. That means you have loss of life, but it’s in the dozens or maybe 100 or 200. If the same earthquake were to happen under a city of that size in a developing country, the number of deaths would be in the thousands if not tens of thousands. Turkey, for example, had great building codes but that didn’t keep tens of thousands of people from getting killed in the 1999 Izmit 7.6-magnitude earthquake because they weren’t paying attention to those codes.
http://www.scientificamerican.com/article.cfm?id=new-zealand-earthquake-christchurch
http://www.gns.cri.nz/
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